The present invention relates generally to printers or printing devices in which printing hammers stamp or strike against types over a sheet of paper thereby to carry out printing. More particularly, the invention relates to a printing device in which a hammer lock releasing mechanism for unlocking each printing hammer from its locked state thereby to permit its printing stroke is capable of carrying out unlocking and locking operations at high speed.
Heretofore, there has been a printer of a kind wherein, as seen in U.S. Pat. No. 3,848,527, a plurality of hammers are grouped into sets and, for each set, there are provided a mechanism for locking the hammers in non-printing positions, an electromagnet for unlocking the locking mechanism, and a roller having, for each hammer, a hammer rotation permitting part and a hammer resetting part. The hammer rotation permitting parts and hammer resetting parts of this roller are disposed at positions in the rotational direction differing from hammer to hammer. The hammers of each set are simultaneously released by the operation of the electromagnet from their locked states due to the locking mechanism, and, in accordance with the rotation of the roller, the hammers which are caused to confront the hammer rotation permitting parts of the roller are permitted to rotate thereby to print. In a printer of this character, there is no necessity of providing an electromagnet for each hammer, one electromagnet being sufficient for a plurality of hammers of each set, whereby the construction of the device becomes simple.
The locking mechanism in this known printer comprises an armature which engages one part of a rotating lever of each hammer and, when the electromagnet is energized, is attracted thereto thereby to rotate and a spring for urging this armature into the above mentioned locked state. The armature is attracted toward the electromagnet when it is energized and, overcoming the force of the spring, rotates to release the rotating lever of the hammer from its locked state. When the electromagnet is not being energized, the armature is rotated in the reverse direction by the force of the spring to engage the rotating lever and thereby to lock the hammer in its non-printing position.
In order to increase the return speed of the armature when the electromagnet is not being energized in this known printer for the purpose of speeding up the entire printing operation, the force of the return spring must be increased. However, when the force of the spring is increased, the size of the electromagnet for rotating the armature against this spring force must also be increased, and the electric power consumption also increases. On the other hand, when an electromagnet of low capacity is used in order to reduce the electric power consumption, the above mentioned force of the spring must be reduced in order obtain positive armature attrating operation. In this case, the return motion of the armature when the electromagnet is deenergized becomes slow, and the total result is that the printing speed cannot be increased.